1. Field of the Invention
The invention relates to a solid electrolytic capacitor using niobium or a niobium alloy for an anode, and a method of manufacturing the same.
2. Description of Related Art
Along with recent miniaturization of electronic equipment such as personal computers, solid electrolytic capacitors to be used for such electronic equipment are also desired to be small and have high capacity. In view of such desire, solid electrolytic capacitors are under development that use, for a dielectric layer, niobium oxide formed by anodizing an anode made of niobium (called a niobium solid electrolytic capacitor, below), instead of tantalum oxide formed by anodizing an anode made of tantalum. This is because niobium oxide has a dielectric constant that is approximately 1.8 times larger than that of tantalum oxide, and hence the niobium solid electrolytic capacitor can achieve an increase in capacity compared with that with the aforementioned solid electrolytic capacitor using tantalum oxide (called a tantalum solid electrolytic capacitor, below). Accordingly, the niobium solid electrolytic capacitor has an advantage that it can be smaller in size than the tantalum solid electrolytic capacitor in the case of obtaining comparable capacities, and is therefore expected to serve as a next-generation solid electrolytic capacitor.
However, niobium solid electrolytic capacitors under development have a problem regarding equivalent series resistance (called ESR, below) and leakage current. For practical use of such niobium solid electrolytic capacitors, further reduction in ESR and leakage current is desired.
To reduce ESR of a solid electrolytic capacitor, a technique has been disclosed in which, instead of conductive polymers used as a cathode, a metal layer including silver, platinum or gold is directly formed on a surface of a dielectric layer made of a metal oxide film formed by anodizing an anode made of valve metal particles (see, for example, Japanese Patent Application Publication No. 2004-319971 (called Patent Document 1, below)).
However, the technique described in Patent Document 1 is aimed to reduce ESR, and not to reduce leakage current. For this reason, it is confirmed that the solid electrolytic capacitor described in Patent Document 1, using niobium oxide as the dielectric and platinum as the cathode, for example, cannot sufficiently reduce leakage current.